The invention relates to a hydrostatic drive and more particularly to such a drive for road trucks and omnibuses, comprising two adjustable hydrostatic machines connected to each other by two branches of a duct and of which the one is adapted to act as a pump and the other as a motor dependent on the mode of operation, pressure limiting and check valves for connecting the two duct branches, and a feed pump and a storage tank, said feed pump being connected with said tank for the supply of hydraulic fluid therefrom.
Generally speaking such a drive is suitable for all road and railroad vehicles and is more particularly to be used with road trucks and omnibuses, although it may also be utilized in elevator and like installations for instance.
In the case of a hydrostatic drive of this type employed as a vehicle transmission one respective hydrostatic machine is connected with the drive engine and also with the vehicle running gear, that is to say the driven axle or axles, either permanently or via a clutch that may be disconnected. Such a hydrostatic machine is for instance described in the German Pat. No. 2,904,572 and may function either as a pump or as a motor dependent on the particular construction involved. Furthermore, in some cases, such a hydrostatic machine may be adjusted to determine which of its two connections functions as a high pressure connection and which functions as a low pressure one.
In the case of use with a differential drive (such as a standard or planetary differential), the manner of operation of the hydrostatic machine may be selectively set as pump or motor operation in accordance with the function required so that the connection between the hydrostatic machines and the engine or the driven axles takes place indirectly in this case.
The two hydrostatic machines are connected with each other by two power branches. During normal, unbraked running the hydrostatic machine connected with the engine, for example, runs as a pump supplying a hydraulic fluid along the duct branch at a high pressure so that owing to its displacement the fluid drives the other hydrostatic machine acting as a hydraulic motor.
In the hydrostatic machine on the output side, the pressure of the hydraulic fluid is converted, such fluid returning via a second duct branch at a low pressure back to the drive hydrostatic machine.
The high and low pressure ducts have a pressure limiting valve between them which opens and produces a connection, when the pressure in the high pressure duct exceeds a certainn value, in order to prevent damage to the hydraulic drive by any excessive pressures which may occur. Furthermore check valves are necessary via which hydraulic fluid may be fed into the duct system of the drive from a feed pump.
More particularly when they are heavily loaded, it is necessary for road trucks and omnibuses to use a sustained action brake in order to brake the vehicle as evenly as possible in certain driving situations, more especially when descending hills, over extended periods of time.
So far so-called engine brakes have been mainly used for this purpose in which the pumped power dissipation of a diesel engine is employed. Furthermore, eddy current brakes have been used.
The shortcoming with such known sustained action brakes is that they not only form a separate and expensive item of the equipment of the vehicle, but furthermore that their effect is comparatively difficult to regulate. In the case of gentle hills it may be that known types of such brakes give a stronger braking action than is desired by the driver. In such a case the sustained action brake will be switched on and off so that the speed of the vehicle is so irregular as to reduce the comfort of vehicle occupants.